A more gradual approach involves vast reflective sails propelled by sunlight or by lasers beamed from the Solar System - wikipedia 
A solar sail concept - wikimedia 
This concept overlaps with the **Breakthrough Starshot** project. For a generation ship, sails could provide continuous low acceleration for decades, eventually achieving significant velocity. - **Advantages:** No onboard fuel requirement. - **Challenges:** Requires massive, fragile sails and constant energy from home-based lasers.